Glass is the most important material in optics, with uses in optical fibers, lenses, mirror substrates, and prisms. The silicate family, the prime material in this article, is well-suited for passive optical functions such as light guidance in a fiber. Because of its many desirable properties, one is tempted to use glass in active functions as well—such as in amplification and modulation of light. As early as the beginning of the 1960s, glasses doped with rare-earth ions were used as gain media for lasers. Nd:glass lasers and Erdoped fiber amplifiers are two examples of the success of the use of doped glass to perform an active optical function. The modulation and switching of light proved to be a more difficult task to perform with glass systems. Glass has very low optical nonlinearity, and the application of an electric field does little to the optical wave traveling in the material.
In 1991 Myers, Mukherjee, and Brueck submitted a polymer film on a silica substrate to a poling process, which consists of the application of high voltages at a temperature of ~300°C. After cooling to room temperature with the voltage still applied, the poled sample was illuminated with a strong infrared (ir) laser beam. As expected they found that material frequency-doubled the incoming radiation and some green light could be measured at the output. When a test was carried out to discard any contribution from the substrate to the frequency-doubled light, they discovered that it was the silica glass that was generating the green light and that they had induced a strong optical nonlinearity in silica by thermal poling. Since then poling of glasses in the context of nonlinear optics has been the subject of many publications, including the report that ultraviolet (uv) radiation can advantageously replace thermal excitation.